The present invention relates to a printed article which is useful as a printed article for OHP or a high gloss printed article, of which a toner image is formed by electrophotography, and to a production method of the same.
In a conventional image forming apparatus, a photoreceptor as a latent image carrier such as a photosensitive drum or a photosensitive belt is rotatably supported to the main body of the image forming apparatus. During the image forming operation, a latent image is formed onto a photosensitive layer of the photoreceptor and, after that, is developed with toner particles to form a visible image. Then, the visible image is transferred to a receiving medium. For transferring the visible image, there is a known method of directly transferring the visible image to the receiving medium by using a corona transfer or a transfer roller and another known method of first transferring the visible image to an intermediate transfer member such as a transfer drum or a transfer belt and then transferring it to the receiving medium.
Such methods are employed in monochrome image forming apparatuses. In addition, for a full-color image forming apparatus having a plurality of photoreceptors and developing devices, there is a known method transferring a plurality of color images on a transfer belt or a transfer drum to a receiving medium such as a paper in such a manner that the respective color images are sequentially superposed on each other, and then fixing these images. The apparatuses according to such a method using a belt are categorized as a tandem type, while the apparatuses according to such a method using a drum are categorized as a transfer drum type. Moreover, an intermediate transferring type is also known in which respective unicolor images are sequentially primary-transferred to an intermediate transfer member and the primary-transferred images are secondary-transferred to a receiving medium such as a paper at once.
Recently, transparent substrates are used as receiving media and light transmitting images are formed on such transparent substrates for projecting images on a screen using an overhead projector (OHP). Attempts have been made to form light transmitting images on transparent substrates by the electrophotography to produce such a printed article for OHP. In practice, however, there is a problem that a projected image of such a printed article for OHP is not clear because of low transparency of a toner image of the printed article and another problem that desired colors is not exactly reproduced in the image projected on the screen such that the tone of projected image is darkened compared to the tone of the toner image on the printed article for OHP. This is because the irregularity of outer surface of a toner image makes scattering or irregular reflection of lights to reduce the transmission of lights on the images so that sufficient quantity of light hardly reaches the projecting screen. This is one of the factors reducing the visibility and the transparency of the projected image. The reduction in visibility and transparency is a phenomenon exhibited particularly on a printed article for OHP using two or more colors.
Japanese Patent Publication No. H05-62342 is directed to solve the aforementioned problems and discloses that the surface roughness of a toner image is smoothed by coating the surface of the toner image with a transparent lacquer, attaching a transparent adhesive tape on the surface of the toner image, or applying heat or pressure to change the shapes of toner particles. From the description in this publication that heat or pressure is applied to the toner image for smoothing the roughness of the surface, it is found that the toner particles must be flexible and have high fluidity during fixing process. In order to prevent “filming phenomenon” of such toner particles onto a fixing roller, the fixing roller is lubricated with oil to have a resistance against toner offset at high temperature. As the fixing roller is lubricated with oil, however, the adhesive property of the transparent adhesive tape is affected so as to produce floating of a toner image in the long run, resulting in reduction in visibility and transparency of the projected image. With a toner image which is flexible and has high fluidity, a fixing roller used must have a soft outer layer for smoothing the surface of the toner image. Such fixing roller can envelope entirely the toner image when touching and thus can evenly apply thermal energy to the toner image. However, the soft fixing roller has poor durability and poor heat conductivity, so the soft fixing roller is unsuited for mass high-speed printing. In addition, when the surfaces of flexible toner particles are smoothed by applying heat or pressure, “shine” is produced on toner image. The shine makes reading of characters difficult.
On the other hand, with oilless fusing toner which can eliminate the necessity of using oil for coating the fixing roller, the filming phenomenon of toner particles to the fixing roller is prevented and resistance against toner offset at high temperature is obtained. However, the amount of wax in the toner particles is large so that the transparency is affected by solidifying property of wax, thus affecting projected images.
Besides the printed articles for OHP, attempts have been made to form an image on a non-transparent sheet with gloss as a receiving medium by employing the electrophotography to produce a high gloss printed article. Also in this case, the irregularity of outer surface of a toner image makes scattering or irregular reflection of lights to reduce the reflection of lights on the images, thus increasing difference in gloss level between the gloss level of the image and the non-transparent sheet. Such increased difference in gloss level is defects of high gloss printed articles.
In case of using oilless fusing toner, a process is taken for reducing the fluidity during fixing. The process may comprise increasing the amount of wax in toner particles or adding cross-linking component to a resin as a thermoplastic resin. The surface of the toner image after fixed includes a solidified lump of wax and has increased irregularity because the toner image is hard, thus increasing difference in gloss level between the image and the non-transparent sheet. That is, there is the same problem in case of using oilless fusing toner. The increased difference in gloss level is a phenomenon exhibited particularly on a high gloss printed article using two or more colors and only permits the production of printed articles having poor color saturation.
The first object of the present invention is to provide a printed article suitable for OHP of which a toner image is easily read and a projected image has excellent visibility and transparency and to provide a production method of the same.
The second object of the present invention is to provide a printed article suitable as a high gloss printed article of which a toner image has a gloss level nearly equal to that on a non-transparent sheet and having improved color saturation and to provide a production method of the same.